The National Security Agency is searching the contents of vast amounts of Americans’ e-mail and text communications into and out of the country, hunting for people who mention information about foreigners under surveillance, according to intelligence officials.

The N.S.A. is not just intercepting the communications of Americans who are in direct contact with foreigners targeted overseas, a practice that government officials have openly acknowledged. It is also casting a far wider net for people who cite information linked to those foreigners, like a little used e-mail address, according to a senior intelligence official.